This invention relates to a variable light transmittance glass board which is functional as an electrochromic cell. More particularly, the glass board is comprised of two transparent glass substrates each of which carries a transparent electrode layer such that the two transparent electrode layers are held opposite to and spaced from each other, first and second electrochromic layers formed on the two transparent electrode layers, respectively, by using tungsten trioxide for the first electrochromic layer and Prussian blue for the second electrochromic layer, and an electrolyte liquid which fills up the gap between the first and second electrochromic layers.
We have disclosed an electrochromic cell of the above construction in Japanese patent application primary publication No. 59-155833 (1984) and corresponding U.S. patent application Ser. No. 696,922 filed Jan. 31, 1985. Usually the electrolyte liquid is a solution of a supporting electrolyte such as a lithium salt in an organic polar solvent such as propylene carbonate. In this electrochromic cell the first electrochromic layer using tungsten trioxide assumes a blue color in its electrochemically reduced state, whereas the second electrochromic layer using Prussian blue assumes a deep blue color in its electrochemically oxidized state.
Such an electrochromic cell is useful as a display device and, besides, can be used as a glass board of which the transmittance of light is variable by coloring and bleaching of the electrochromic layers. If the variable light transmittance glass board is to be used in automobiles for the instrument panel, sun-roof panel, side windows or sun visors, the glass board is required to exhibit good durability as an electrochromic cell even at elevated temperatures. According to some manufacturers' standards it is required that after subjecting the variable transmittance glass board to accelerated aging at 90.degree. C. for 1000 hr the quantity of electricity transferred at coloring and bleaching of the electrochromic layers be not less than 60% of the initial value measured on the same sample in fresh state. However, according to our experiments the heating at 90.degree. C. for 1000 hr often causes the quantity of electricity transferred at the coloring and bleaching to decrease to only about 35% of the initial value, so that the variable transmittance glass board becomes poor in its shading capability.
There is another problem in the same electrochromic cell. In producing this cell, the tungsten trioxide layer and the Prussian blue layer are both formed in electrochemically oxidized state. Therefore, it is necessary to electrochemically reduce one of the electrochromic layers before using the cell in order that the two electrochromic layers may simultaneously assume color and simultaneously bleach. This reduction treatment is called the initial reduction. As the initial reduction is performed there occurs partial decomposition of the electrolyte liquid on the surface of the opposite electrochromic layer by reason of existence of inevitable impurities. The problem arises from this phenomenon. Since bubbles are formed by such decomposition of the electrolyte liquid, the appearance of the electrochromic cell or variable transmittance glass board is marred.
As a solution to this problem, Japanese patent application primary publication No. 59-159134 (1984) proposes an electrochromic cell of the above described type characterized by the addition of at least one auxiliary electrode, which comprises an electrochemically oxidizable and reducible material and is disposed in a marginal region of the cell. At the initial reduction of one electrochromic layer the auxiliary electrode is used as a counter electrode. Then the electrochemically active material in the auxiliary electrode undergoes oxidation so that the decomposition of the electrolyte liquid on the surface of the other electrochromic layer is avoided. However, in a large-sized electrochromic cell useful as a variable light transmittance glass board the effect of the auxiliary electrode will be insufficient unless the active material in the auxiliary electrode is very large in the quantity of electricity it gains per unit volume at its oxidation or the auxiliary electrode is made large in size. The auxiliary electrode materials named in Japanese No. 59-159134 are not sufficiently large in the quantity of electricity per unit volume. On the other hand, it is unfavorable to enlarge the size of the auxiliary electrode because of a decrease in an effective transmissive area of the electrochromic cell or variable transmittance glass board.